To make an executable program, the GHC system compiles your code and then links it with a non-trivial runtime system (RTS), which handles storage management, profiling, etc.
You have some control over the behaviour of the RTS, by giving special command-line arguments to your program.
When your Haskell program starts up, its RTS extracts
command-line arguments bracketed between
+RTS
and
-RTS
as its own. For example:
% ./a.out -f +RTS -p -S -RTS -h foo bar
The RTS will snaffle -p
-S
for itself, and the remaining arguments -f -h foo bar
will be handed to your program if/when it calls
System.getArgs
.
No -RTS
option is required if the
runtime-system options extend to the end of the command line, as in
this example:
% hls -ltr /usr/etc +RTS -A5m
If you absolutely positively want all the rest of the options
in a command line to go to the program (and not the RTS), use a
––RTS
.
As always, for RTS options that take sizes: If the last character of size is a K or k, multiply by 1000; if an M or m, by 1,000,000; if a G or G, by 1,000,000,000. (And any wraparound in the counters is your fault!)
Giving a +RTS -f option will print out the RTS options actually available in your program (which vary, depending on how you compiled).
NOTE: since GHC is itself compiled by GHC, you can change RTS options in the compiler using the normal +RTS ... -RTS combination. eg. to increase the maximum heap size for a compilation to 128M, you would add +RTS -M128m -RTS to the command line.
RTS options are also taken from the environment variable
GHCRTS
. For example, to set the maximum heap size
to 128M for all GHC-compiled programs (using an
sh-like shell):
GHCRTS='-M128m' export GHCRTS
RTS options taken from the GHCRTS
environment
variable can be overriden by options given on the command
line.
There are several options to give you precise control over garbage collection. Hopefully, you won't need any of these in normal operation, but there are several things that can be tweaked for maximum performance.
-A
size[Default: 256k] Set the allocation area size
used by the garbage collector. The allocation area
(actually generation 0 step 0) is fixed and is never resized
(unless you use -H
, below).
Increasing the allocation area size may or may not give better performance (a bigger allocation area means worse cache behaviour but fewer garbage collections and less promotion).
With only 1 generation (-G1
) the
-A
option specifies the minimum allocation
area, since the actual size of the allocation area will be
resized according to the amount of data in the heap (see
-F
, below).
-c
Use a compacting algorithm for collecting the oldest generation. By default, the oldest generation is collected using a copying algorithm; this option causes it to be compacted in-place instead. The compaction algorithm is slower than the copying algorithm, but the savings in memory use can be considerable.
For a given heap size (using the -H
option), compaction can in fact reduce the GC cost by
allowing fewer GCs to be performed. This is more likely
when the ratio of live data to heap size is high, say
>30%.
NOTE: compaction doesn't currently work when a single
generation is requested using the -G1
option.
-c
n[Default: 30] Automatically enable
compacting collection when the live data exceeds
n% of the maximum heap size
(see the -M
option). Note that the maximum
heap size is unlimited by default, so this option has no
effect unless the maximum heap size is set with
-M
size.
-F
factor[Default: 2] This option controls the amount of memory reserved for the older generations (and in the case of a two space collector the size of the allocation area) as a factor of the amount of live data. For example, if there was 2M of live data in the oldest generation when we last collected it, then by default we'll wait until it grows to 4M before collecting it again.
The default seems to work well here. If you have
plenty of memory, it is usually better to use
-H
size than to
increase
-F
factor.
The -F
setting will be automatically
reduced by the garbage collector when the maximum heap size
(the -M
size
setting) is approaching.
-G
generations[Default: 2] Set the number of generations used by the garbage collector. The default of 2 seems to be good, but the garbage collector can support any number of generations. Anything larger than about 4 is probably not a good idea unless your program runs for a long time, because the oldest generation will hardly ever get collected.
Specifying 1 generation with +RTS -G1
gives you a simple 2-space collector, as you would expect.
In a 2-space collector, the -A
option (see
above) specifies the minimum allocation
area size, since the allocation area will grow with the
amount of live data in the heap. In a multi-generational
collector the allocation area is a fixed size (unless you
use the -H
option, see below).
-H
size[Default: 0] This option provides a “suggested heap size” for the garbage collector. The garbage collector will use about this much memory until the program residency grows and the heap size needs to be expanded to retain reasonable performance.
By default, the heap will start small, and grow and
shrink as necessary. This can be bad for performance, so if
you have plenty of memory it's worthwhile supplying a big
-H
size. For
improving GC performance, using
-H
size is
usually a better bet than
-A
size.
-k
size[Default: 1k] Set the initial stack size for new threads. Thread stacks (including the main thread's stack) live on the heap, and grow as required. The default value is good for concurrent applications with lots of small threads; if your program doesn't fit this model then increasing this option may help performance.
The main thread is normally started with a slightly larger heap to cut down on unnecessary stack growth while the program is starting up.
-K
size[Default: 1M] Set the maximum stack size for an individual thread to size bytes. This option is there purely to stop the program eating up all the available memory in the machine if it gets into an infinite loop.
-m
nMinimum % n of heap which must be available for allocation. The default is 3%.
-M
size[Default: unlimited] Set the maximum heap size to size bytes. The heap normally grows and shrinks according to the memory requirements of the program. The only reason for having this option is to stop the heap growing without bound and filling up all the available swap space, which at the least will result in the program being summarily killed by the operating system.
The maximum heap size also affects other garbage
collection parameters: when the amount of live data in the
heap exceeds a certain fraction of the maximum heap size,
compacting collection will be automatically enabled for the
oldest generation, and the -F
parameter
will be reduced in order to avoid exceeding the maximum heap
size.
-s
file, -S
fileWrite modest (-s
) or verbose
(-S
) garbage-collector statistics into file
file. The default
file is
program.stat. The
file stderr
is treated specially, with the output really being sent to
stderr
.
This option is useful for watching how the storage manager adjusts the heap size based on the current amount of live data.
-t
Write a one-line GC stats summary after running the
program. This output is in the same format as that produced
by the -Rghc-timing
option.
These RTS options might be used (a) to avoid a GHC bug, (b) to see “what's really happening”, or (c) because you feel like it. Not recommended for everyday use!
-B
Sound the bell at the start of each (major) garbage collection.
Oddly enough, people really do use this option! Our pal in Durham (England), Paul Callaghan, writes: “Some people here use it for a variety of purposes—honestly!—e.g., confirmation that the code/machine is doing something, infinite loop detection, gauging cost of recently added code. Certain people can even tell what stage [the program] is in by the beep pattern. But the major use is for annoying others in the same office…”
-D
numAn RTS debugging flag; varying quantities of output
depending on which bits are set in
num. Only works if the RTS was
compiled with the DEBUG
option.
-r
fileProduce “ticky-ticky” statistics at the
end of the program run. The file
business works just like on the -S
RTS
option (above).
“Ticky-ticky” statistics are counts of
various program actions (updates, enters, etc.) The program
must have been compiled using
-ticky
(a.k.a. “ticky-ticky profiling”), and, for it to
be really useful, linked with suitable system libraries.
Not a trivial undertaking: consult the installation guide on
how to set things up for easy “ticky-ticky”
profiling. For more information, see Section 5.7.
-xc
(Only available when the program is compiled for profiling.) When an exception is raised in the program, this option causes the current cost-centre-stack to be dumped to stderr.
This can be particularly useful for debugging: if your program is complaining about a head [] error and you haven't got a clue which bit of code is causing it, compiling with -prof -auto-all and running with +RTS -xc -RTS will tell you exactly the call stack at the point the error was raised.
The output contains one line for each exception raised in the program (the program might raise and catch several exceptions during its execution), where each line is of the form:
< cc1, ..., ccn >
each cci is a cost centre in the program (see Section 5.1), and the sequence represents the “call stack” at the point the exception was raised. The leftmost item is the innermost function in the call stack, and the rightmost item is the outermost function.
-Z
Turn off “update-frame squeezing” at garbage-collection time. (There's no particularly good reason to turn it off, except to ensure the accuracy of certain data collected regarding thunk entry counts.)
GHC lets you exercise rudimentary control over the RTS settings for any given program, by compiling in a “hook” that is called by the run-time system. The RTS contains stub definitions for all these hooks, but by writing your own version and linking it on the GHC command line, you can override the defaults.
Owing to the vagaries of DLL linking, these hooks don't work under Windows when the program is built dynamically.
The function
defaultsHook
lets you change various RTS options. The commonest use for this
is to give your program a default heap and/or stack size that is
greater than the default. For example, to set
-M128m -K1m:
#include "Rts.h" #include "RtsFlags.h" void defaultsHook (void) { RtsFlags.GcFlags.maxStkSize = 1000002 / sizeof(W_); RtsFlags.GcFlags.maxHeapSize = 128*1024*1024 / BLOCK_SIZE_W; }
Don't use powers of two for heap/stack sizes: these are more likely to interact badly with direct-mapped caches. The full set of flags is defined in ghc/rts/RtsFlags.h the the GHC source tree.
You can also change the messages printed when the runtime system “blows up,” e.g., on stack overflow. The hooks for these are as follows:
void ErrorHdrHook (FILE *)
What's printed out before the message from
error
.
void OutOfHeapHook (unsigned long, unsigned long)
The heap-overflow message.
void StackOverflowHook (long int)
The stack-overflow message.
void MallocFailHook (long int)
The message printed if malloc
fails.
void PatErrorHdrHook (FILE *)
The message printed if a pattern-match fails (the failures that were not handled by the Haskell programmer).
void PreTraceHook (FILE *)
What's printed out before a trace
message.
void PostTraceHook (FILE *)
What's printed out after a trace
message.
For example, here is the “hooks” code used by GHC itself:
#include "Rts.h" #include "../rts/RtsFlags.h" #include "HsFFI.h" void defaultsHook (void) { RtsFlags.GcFlags.heapSizeSuggestion = 6*1024*1024 / BLOCK_SIZE; RtsFlags.GcFlags.maxStkSize = 8*1024*1024 / sizeof(W_); RtsFlags.GcFlags.giveStats = COLLECT_GC_STATS; RtsFlags.GcFlags.statsFile = stderr; } void ErrorHdrHook (long fd) { char msg[]="\n"; write(fd,msg,1); } void PatErrorHdrHook (long fd) { const char msg[]="\n*** Pattern-matching error within GHC!\n\nThis is a compiler bug; please report it to glasgow-haskell-bugs@haskell.org.\n\nFail:"; write(fd,msg,sizeof(msg)-1); } void PreTraceHook (long fd) { const char msg[]="\n"; write(fd,msg,sizeof(msg)-1); } void PostTraceHook (long fd) { #if 0 const char msg[]="\n"; write(fd,msg,sizeof(msg)-1); #endif }